The invention relates to a method for determining the critical current density j.sub.c (x, y) and its homogeneity in a superconductor tape and a measuring apparatus for carrying out the method.
The measuring is carried out inductively and without contact in a cryostat, in which the superconductor tape is cooled below its transition temperature.
The state of the technology is a resistive measuring method, for example, with current fed into the superconductor tape and voltage contacts, either at the conductor ends for insulated tape conductors or with attached contacts or voltage taps by way of sliding contacts in the case of non-insulated superconductors. This is referred to as a contacting measuring method.
When compared to an inductive method, the resistive current density measuring has a decisive disadvantage in that the device to be tested must be contacted. Contact measuring is not only very involved, but additionally it is not very reliable since the Bi-2223 tape in particular is frequently coated with an organic layer, which has to be dissolved and causes non-reproducible transition resistances during the tapping. This is not suitable for a technical processing method.
Contactless methods must also be mentioned, that is to say the Hall probe magnetometry, such as is used for Y-123-sputtering films (see W. Xing et al.,"Magnetic Flux Mapping, . . .," in J. Appl. Phs., 76(7), 1994, pp 4244-4255; and M. Polak et al.,"Contactless Measurement of Voltage-Current Characteristics . . .," in Physica C, 174 (1991), pp 14-22); and Bi-2223 single-core conductor tape segments (see Markku Lahtinen et al., "Homogeneity Study of Bi-2223/Ag Monofilamentary Tapes . . .," in Physica C 244 (1995), pp 115-122; A. Tanihata et al., "Scanning Hall Probe Microscopy . . . ," in Supercond. Sci. Techn. 9 (1996), pp 1055-1059; and K. Kawano and A. Oota, "A study on Self-Field Distribution . . . " in Physica C 275 (1997), pp 1-11). For the last-mentioned method, the fields of magnetized devices to be tested are tested with Hall probes. These contactless measuring methods measure the residual field distribution of short stationary samples (in the cm range), following the magnetizing in the external field, and its decay. In order to measure the residual field distribution, the device to be tested is scanned with the Hall probe employing an x-y shift table. The current density distribution is determined from the position-dependent Hall signal with the aid of a computer analysis. A pull-through method is described in Siemens Forsch -u. Entwickl.-Ber. [Siemens Research and Development Report], Volume 5 (1976), No. 2, pp 98-103, for which the magnetic field strength of a screening current is determined outside of the superconductor with Hall probes.